by default, unix (under os x) allows non-owners of a folder to read the contents but not write or execute - in addition, you also get automatically logged into a certain directory (usually your home dir) when you use a username/password combo to connect to a unix box.

if you want to limit a certain account to a certain directory, you can do a few different things - but first, make a new account on the box for them only and give them that username and password - then they'll have a home dir of their own (/Users/[WHATEVER_NAME]. at that point, if you want them to only have access to a another dir on the box, you can:

a. create a shortcut in their home dir to that dir and then they can click on the shortcut

or

b. open the netinfo manager (/Applications/Utilities) and click on users in the middle col - pick their new username and in the bottom panel, change the path from /Users/[WHATEVER_NAME] (where whatever name is the name you gave them) to the path to the desired directory - then when they log in, their home directory is that directory. to find the dir path for any given dir, find it in the finder and do a "get info" on it - the "where" value near the top of that dialog will give you the path...

a is a lot easier and quicker, but you could do b if you really wanted to start them off at a specific dir...